Optical Coherence Tomography (“OCT”) is a type of optical coherence-domain reflectometry that uses low coherence interferometry to perform high-resolution ranging and cross-sectional imaging. In OCT systems, a light beam from a low coherence light source is split into a reference light beam and a sample light beam. The sample light beam is directed onto a sample and the light scattered from the sample is collected and combined with the reference light beam. The combination of the sample and reference light beams results in an interference pattern corresponding to the variation in the sample reflection with the depth of the sample, along the sample beam. The sample beam typically suffers a high loss of energy due to its interaction with the sample. The reference beam serves as a local oscillator used to recover interference information at a detectable level in the return light, and typically must have a much higher energy level than the light beam returned from the sample. For example, the reference beam may have intensity under about 1% of the source, and the light returned from the sample may be substantially weaker.
In OCT, the reference beam and the collected sample beam are mixed on a photo detector, which detects the interference signal. The output of the photodetector is processed to computationally generate a cross-sectional image of the sample. The intrinsic scale of interference effects provides high-resolution (less than 10-20 micrometer) imaging of the cross-sections of the sample, making such OCT useful in biological and medical examinations and procedures, as well as in materials and manufacturing applications.
Examples of such a system have been described in U.S. Pat. No. 6,134,003 (“the '003 patent”), which particularly describes one construction based on an interferometer that includes a broadband optical radiation source; an optical radiation detector; a reference optical reflector; a first optical path leading to the reference optical reflector; and a second optical path including the sample that is to be imaged. A beam divider splits the optical radiation from the optical radiation source along the first optical path to the reference reflector and along the second optical path to the structure being viewed. The optical radiation detector is positioned to receive reflected optical radiation from the reference reflector and reflected optical radiation returning from the structure. Light from these two paths self-interferes, so the detector generates a signal in response to the reflected optical radiation. A processor utilizes the signals from the detector to generate an image of the structure being viewed. The reference optical reflector is typically a mirror coupled to a movable actuator to provide periodic movement to the reference mirror.
The prior art system of the '003 Patent is broken down into a several major subsystems as shown in FIGS. 1, 2A and 2B of that patent, which are reproduced here as FIGS. 1, 2A and 2B. These are described in the '003 patent as follows
“ . . . the imaging system includes an optical radiation source 2, an interferometer 4, a detector 16, and an endoscopic unit 34. The interferometer 4 may be of any of the types known to one skilled in the art. For the purposes of discussion only, the embodiment will be discussed in terms of a Michelson interferometer. However, other embodiments using the other types of interferometers are contemplated. The interferometer 4 of this embodiment includes a beam divider 6, which divides the optical radiation along a first optical path defining a reference arm 8, and a second optical path defining a measuring arm 10. The optical path defining a reference arm 8 includes a reference reflector 12. The optical path defining the measuring arm 10 includes the endoscopic unit 34.
In general, the interferometer 4 operates by transmitting radiation from the optical radiation source 2 to the beam divider 6 where it is divided and transmitted along the optical paths defining the reference arm 8 and the measuring arm 10. Light reflected from the beam divider 6 travels along the reference arm 8 and is reflected back by the reference reflector 12. Light transmitted through the beam divider 6 along the measuring arm 10 travels through the endoscopic unit 34 and illuminates a structure 14 under observation. Light reflected by the structure 14 travels back through the endoscopic unit 34 along the measuring arm 10 to the beam divider 6. The radiation reflected from the reference reflector 12 and the radiation reflected from the structure 14, is then recombined by the beam divider 6 and transmitted to the detector 16. The resulting combined radiation generates an interference pattern at the detector 16, which typically generates electrical signals representative of the combined radiation and transmits these signals to signal processing and control electronics and display unit 18 where an image of the structure is obtained and analyzed.
By changing the length of the reference arm 8, longitudinal scanning is accomplished. Longitudinal scanning provides a way of changing the location at which interference in the optical radiation being reflected from the structure 14 back through the endoscopic unit 34 is detected. If the optical radiation is emitted off axis to the longitudinal axis of the endoscopic unit 34, such scanning provides a means of viewing different tissue depths. In one embodiment, the length of the reference arm 8 is changed by moving the reference reflector 12.
By rotating the optical radiation beam emitted from the endoscopic unit 34, rotational scanning may be accomplished. In rotational scanning, a circumferential path whose radius is centered at the longitudinal axis of the endoscopic unit 34 is viewed.”
Considering each component in more detail, the optical source 2 has characteristics such as wavelength, power, coherence length, and autocorrelation function which are important factors in system performance. In some applications, near infrared sources (1.0-2.0 um) tend to penetrate deeper into many biological media than visible wavelengths and are therefore preferable. The optical radiation source 2 can include in various embodiments.”
And referring to FIGS. 2A and 2B of the '003 patent, that patentee reports
“There are several varieties of interferometers that may be used in the system . . . . Although bulk optical and free space implementations are shown in these figures, there exist equivalent embodiments employing optical fibers” as, for example, shown in the U.S. Pat. No. 5,321,501, U.S. Pat. No. 5,459,570 and U.S. Pat. No. 5,465,147. “One embodiment employs a simple Michelson Interferometer 104, as shown in FIG. 2A. In another embodiment, as shown in FIG. 2B, the interferometer 204 includes a sample reference reflector 213 in the measuring arm 210. The use of this reference reflector 213 in the measuring arm 210 allows for long displacements between a beamsplitter . . . and the sample.
Although faster scanning helps eliminate motion induced artifacts, in most living biological tissues there is a limit to how fast scanning can be accomplished due to the finite signal power that can safely be delivered to the specimen or practical considerations in mechanical scanning systems. Signal processing techniques can help eliminate any residual motion induced artifacts . . . . As shown in the interferometer 204 of FIG. 2B, by placing a sample reference reflector 213 near or on the structure, a differential measurement between the sample reference reflector and structure is possible.
This measurement is less sensitive to any path length variations along the measurement arm 210. In fact the distance to the structure 14 can be made very large. In order to maintain sensitivity, the sample reference reflector 213 must reflect enough radiation to maintain shot-noise-limited operation. The sample reference reflector 213 can be located at the distal end of the endoscopic unit 34 to help overcome potential artifacts caused by the delivery optics.”
OCT based systems may be implemented with fiber optics, and an optical fiber carrying the sample light beam may be incorporated into a catheter or an endoscope for insertion into an internal body cavity or organ, such as a blood vessel, the gastrointestinal tract, the gynecological tract or the bladder, to generate images cross-sections of tissue inside the cavity or organ. The sample beam is typically emitted from the distal end of the instrument, where a prism or a mirror, for example, directs the sample light beam towards a wall of the cavity. The optical fiber and the prism or mirror may be rotated by a motor to facilitate examination of the circumference of the cavity. In OCT systems, either the reference light beam or the sample light beam may be modulated to provide a relatively low frequency beating used as a carrier frequency. Mechanical motion may be used to scan the optical path, which essentially represents the sample depth. This motion also creates a Doppler frequency shift. The amplitude of the frequency of modulation is modulated by the intensity of the reflected and scattered light in the sample beam. The signal is then processed using a narrow band amplifier tuned to the frequency, to extract the intensity variation to produce an image.
The signal processing of these prior art systems consists primarily in extracting the interferometric signal corresponding to the location in the sample at which interference in the optical radiation being returned from the sample is detected. The interferometric signal is substantially proportional to the signal returned from the sample since the reference signal can be considered as a constant. Longitudinal scanning in the reference optical path, which changes path delay, corresponds to scanning the depth dimension of the region being imaged. Such scanning is typically achieved, for example, by moving the reference reflector. As the path length along the axial direction is thus varied, the signal processing electronics creates an image of the sample structure along the longitudinal axis and this is displayed by a suitable display unit (see, for example, the signal traces of FIGS. 7A, 7B and 7C in U.S. Pat. No. 5,459,570). The detector and signal processing electronics may be selected to provide high sensitivity and high dynamic range.
One limit to the sensitivity of the system is dictated by quantum mechanical effects in the detectors. The minimum resolvable reflection from the sample is proportional to the longitudinal velocity V of the reference mirror and the incident source signal power P. Thus, as scan rate increases, greater signal power is needed to maintain a given receiver-sensitivity, and the system should therefore be optimized to enhance signal detection. For example, to achieve this sensitivity, a low noise transimpedance amplifier and sufficient reference signal power may be used to ensure that the shot-noise from the reference arm power dominates the thermal noise of the transimpedance amplifier. Various signal processing approaches may enhance the recoverable information. For example, the reference light beam or the sample light beam may be modulated to provide a relatively low frequency beating used as a carrier frequency, and the processor may synchronously demodulate the signal to extract an envelope corresponding to the interference intensity as described above. The signal processing electronics can also employ phase sensitive detection techniques and inverse scattering theory or bandwidth expansion techniques to extract enhanced resolution or other signal information. One method to enable phase sensitive detection is for the electronic processing unit to consist of an anti-aliasing low pass filter followed by an A/D converter. Suitable digitizing may be effected, for example with a 12- to 16-bit device running at about twice the intermediate frequency. It has further been proposed to extract velocity data from the received signal when the imaged region contains moving scatterers, such as blood cells, by extracting Doppler frequency information from the return signals, e.g., with a spectrum analyzer. The velocity of the imaged scatterers is then detected, with a spatial resolution about equal to the coherence length. This technique is useful tool for analyzing moving blood or flowing bodily fluid (secretions), pulse rate, etc. A digital signal processing unit (DSP) unit may be used to perform such frequency analysis in several ways to simply display the derived velocities, for example by implementing a bank of bandpass filters around a nominal zero Doppler frequency signal.
The foregoing description is believed to accurately represent the state of the art in endoscopic OCT imaging and its common variations.
When compared to conventional forms of optical imaging, OCT will be seen to have several limitations. It requires splitting of a small portion of the source beam to provide a coherent reference beam, and the intensity of this portion governs several aspects of the quality achievable in the image. Moreover, since image construction depends on phase differences between light returned from the tissue and this reference beam, conventional OCT is unable to image with different, or secondary return light, such as tissue fluorescence or dye-enhanced secondary emissions. This is a limitation for many diagnostic requirements. Additionally, tissue should be in within a length of longitudinal scanning or otherwise it would require changing of this scanning to adapt the system to the various distances to the tissue being imaged. It is especially difficult to achieve when the light is brought in and out of the tissue through the fiber optic means (see for example, Iftimia N, Bouma B E, Tearney G J, “Adaptive ranging in cardiovascular OCT imaging”, SPIE Proceeding Vol. 5316, Coherence Domain Optical Methods and Optical Coherence Tomography in Biomedicine VIII, San Jose, Calif., 2004). The difference in the index of refraction, polarization and other optical properties of the tissue and their variations during measurements relative to the reference arm requires additional complexity in order to reduce the effect of theses variations.